This invention generally relates to a frequency converting system and, more particularly, to a double band frequency converting system in a microwave radio transmission system which converts high frequency signals of the microwave band to lower frequency bands.
Generally, the term microwave represents waves having wavelengths measured in centimeter units of which the frequency band usually covers the range of from 300 MHz to 30 GHz and the microwave radio communication system signifies a communication system utilizing microwaves propagated in the atmosphere.
Among the components constituting a microwave radio receiver of the above mentioned microwave radio communications system, the frequency converting system is an apparatus that converts the 3.about.30 GHz microwaves received by the receiver to more easily demodulatable frequencies of 300 MHz.about.3 GHz, i.e., the ultra high frequency wave band or the very high frequency wave band of 50 MHz.about.300 MHz and even lower frequencies.
A double band frequency converting apparatus is generally an apparatus which converts transmitted microwaves of two different frequencies propagated from a microwave frequency oscillator of two different radio frequencies to a lower frequency band, as described hereunder in detail with reference to FIG. 1.
FIG. 1 shows the circuit diagram of a frequency converter which converts high frequency signals of the microwave band propagated from two satellites orbiting over Europe, i.e., the ECS (European Communication Satellite) of 10.95.about.11.7 GHz and Telecom-1 of 12.5.about.12.75 GHz.
In FIG. 1, a low noise amplifier 20 low-noise amplifies the high frequency signal supplied through a wave guide 10.
A first frequency converting section 30 has a first filter 31 for filtering out high frequency signals of the 10.95.about.11.7 GHz band from outputs of the low noise amplifier 20, a first local oscillator 33 for generating a locally oscillated frequency of around 10 GHz and a first mixer 32 for mixing both outputs of the first filter 31 and the first local oscillator 33 to generate an intermediate frequency of 1.about.1.75 GHz.
A second frequency converting section 80 also has a second filter 81 for filtering out high frequency signals of the 12.5.about.12.75 GHz band from outputs of the low noise amplifier 20, a second local oscillator 83 for generating a locally oscillated frequency of around 11.5 GHz, and a second mixer 82 for mixing both outputs of the second filter 81 and the second local oscillator 83 to generate intermediate frequency of 1.about.1.25 GHz.
An intermediate frequency amplifier 21 (referred to as IF amplifier) amplifies the output of the first frequency converting section 30 or the second frequency converting section 80 which is selectively supplied by a first switch SW1, and then supplies the amplified frequency to an indoor TV receiver (not shown) via signal/power transit section 60 and cable connection terminal 40. The frequency converting system connected to the outdoor antenna, that is, the wave guide 10, is connected to the tuner of the indoor TV receiver (not shown) through the cable connection terminal 40.
A power supply converting section 50 comprises a first voltage transformer 51, a second voltage transformer 52 and a third voltage transformer 53, wherein the first voltage transformer 51 transforms DC voltage supplied through both the cable connection terminal 40 and the signal/power transit section 60 to thereafter supply it to the low noise amplifier 20, and the second one 52 transforms the DC voltage to thereafter supply it to the first local oscillator 33 or to the second local oscillator 83 while the third voltage transformer 53 transforms the DC voltage to thereafter supply it to the IF amplifier 21.
A signal/power transit section 60 includes a capacitor 61 which is connected between the IF amplifier 21 and the cable connection terminal 40, and a radio frequency choke coil (hereinafter, referred to as a RF choke coil) 62 which is connected between cable connection terminal 40 and the power supply converting section 50.
An intermediate frequency selection section 70 includes a first switch SW1 which selectively switches on the output of the first mixer 32 or the output of the second mixer 82 to the IF amplifier 21, a second switch SW2 which selectively switches on the output of the second voltage transformer 52 to the first local oscillator 33 or the second local oscillator 83 and a switching controller 71 which controls the first and second switch SW1, SW2 according to the electric potential of the output voltage of the RF choke coil 62.
For converting high frequency signals of two microwave bands to lower frequency bands, the conventional double band frequency converter structured as above, however, add technical problems to the manufacturer who must design two local oscillators which generate two locally oscillated high frequencies slightly different from each other, and two mixers which operate in a high frequency band.
Accordingly, it is an object of this invention to provide a double band frequency converter which minimizes the technical difficulties and can reduce production cost in manufacturing a microwave radio receiver.
In order to accomplish the object of the present invention, a microwave receiver having a wave guide and a low noise amplifier for low-noise amplifying the high frequency signals from two distinct microwave bands received through wave guide comprises according to this invention:
a first frequency converter for converting frequency of the output of the low noise amplifier;
a second frequency converter for converting frequency of the output of the first frequency converter again;
an intermediate frequency amplifier for amplifying the output of the first frequency converter or the second frequency converter as selected; and
an intermediate frequency selector for transmitting the user selected output of either the first or second frequency converter to the intermediate frequency amplifier, whereby the second frequency converter operates in a frequency band that is lower than that for the operation of the first frequency converter.